For the production of paper and board, the use of shoe presses, i.e. presses with an extended nip, has become increasingly widespread. In such a press, a press nip is formed between a rotating roll and a concave shoe whereby a flexible endless band or belt runs around the concave shoe. The earliest shoe presses were of the so called open type where the flexible endless belt was not closed at its edges. This caused problems since a lubricant must be supplied between the shoe and the flexible belt. For this reason, an open shoe press entails the problem of lubricant leakage that results in a high consumption of lubricant as well as a risk that the paper web comes into direct contact with the lubricant, which is usually oil. Modern shoe presses are therefore usually manufactured in the shape of so called enclosed shoe presses.
In an enclosed shoe press, a press roll is used that comprises a concave pressure shoe and two gables or end walls. The endless flexible band consists of a tubular jacket impervious to liquid that runs over the concave shoe and which is fixed at its edges to peripheral parts of the gables. The tubular jacket is thereby closed at its ends such that the problems associated with leakage of the lubricant are eliminated. The fixing to the gables of the edges of the jacket is thus an essential feature of the enclosed shoe press and a number of different solutions for the fixing of the edges of the jacket have been proposed, see for example U.S. Pat. No. 4,975,152 to Filzen et al., U.S. Pat. No. 5,011,578 to Lange et al and U.S. Pat. No. 5,098,523 to Ilmarinen et al.
In Finnish Patent No. 90120 (Tampella Papertech OY), an enclosed shoe press roll is disclosed where the jacket is squeezed by an elastic clamping ring against a stop ring whereby the elastic clamping ring is compressed between an inner or an outer clamping ring of metal and a peripheral gable member and expands radially outward to form a mechanical joint that locks the flexible jacket of the shoe press roll to the gable of the shoe press roll. In Finnish Patent No. 90120, two different types of elastic rings are proposed. According to a first embodiment, an elastic clamping ring with a substantially rectangular cross section is proposed. According to a second embodiment, an elastic clamping ring with a substantially V-shaped cross section is proposed.
U.S. Pat. No. 5,904,813 issued May 18, 1999 and assigned to the assignee of the present application discloses an enclosed shoe press roll which is an improvement over the previously mentioned press roll. A liquid-impervious jacket is secured by the compression action of a rectangular elastic clamping ring as the elastic ring is made to expand radially outward.
Tests have shown that the proposed rectangular cross section is capable of fulfilling its function. However, it has been found that repeated loadings, or loading during long periods of time, can cause a permanent deformation of the rectangular clamping ring such that the cross section of the ring passes from a rectangular shape to a shape that can be described as mushroom-shaped or "chef's-cap shaped". As a result of this phenomenon, problems can arise when the jacket is to be replaced since the elastic clamping ring does not return to its original shape but remains in a position where it still clamps the jacket. Moreover, the rectangular cross section disclosed in Finnish Patent No. 90120 is, as shown in the drawings, shaped in such a way that it has greater height than width. This entails the disadvantage that the elastic clamping ring must be subjected to a relatively high loading in the axial direction of the shoe press roll in order for the ring to be sufficiently deformed to clamp the flexible jacket of the press roll.
The proposed V-shaped cross section can be described in terms of two inclined legs separated by a groove in the ring whereby the groove in the ring faces inwardly. Such a cross section of the elastic clamping ring entails two disadvantages. First, the groove in the ring makes the deformation of the ring during loading more difficult to predict and control. Second, the bottom of the groove is a risk from the point of view of strength and this profile is therefore not suitable in connection with the relatively high loadings the ring will be subject to when it is used.